This invention relates to a method for the enrichment or separation of lithium isotopes, .sup.6 Li and .sup.7 Li, by means of a laser.
In nature, the lithium isotopes, .sup.6 Li and .sup.7 Li, exist at a proportion of 7.42:92.58.
Recently, .sup.6 Li has come to arrest increasing attention as the source for tritium, T, which is used as the fuel for thermonuclear fusion furnaces. In other words, the importance of tritium T as the fuel for thermonuclear fusion furnaces has come to be recognized in consequence of the growth of researches on thermonuclear fusion furnaces. On the earth, tritium T is extremely rare and exists in a proportion of only 1/10.sup.18 to 1 with respect to ordinary hydrogen (protium). In view of such extremely meager natural presence of tritium, it is readily appreciated that the isotope must be artificially manufactured by some method or other in order for the isotope to be used as a fuel on a commercial scale. One of the simplest methods so far available is by a nuclear reaction of the following formula which is caused by irradiation of .sup.6 Li with thermo-neutrons. This nuclear reaction permits ready production of tritium T. EQU .sup.6 li+.sup.1 n.fwdarw..sup.3 T+.sup.4 He +4.8 MeV
also, tritium T can be produced from .sup.7 Li by a reaction of the following formula. In this case, the reaction does not occur unless the neutrons to be used have a sufficiently high energy level. For this reason, .sup.7 Li is mainly used as the absorbent for neutrons in nuclear fission furnaces. EQU .sup.7 Li +.sup.1 n.fwdarw..sup.3 T+.sup.4 He+.sup.1 n-2.5 MeV
to be more specific, .sup.6 Li is used as the source for the initial charge of tritium in thermonuclear fusion furnaces or as the breeder for tritium within the blankets of such furnaces. In contrast, .sup.7 Li concentrated to more than 99% is used as the absorbent for neutrons and as the pH adjuster for preventing hydrogen embrittlement in nuclear fission light-water furnaces. In a nuclear fission power generation furnace having a capacity of 550,000 KW, for example, about 70 Kg of .sup.7 Li is consumed for this purpose per year.
As described above, the technique adopted for the enrichment or separation of the isotopes of lithium, .sup.6 Li and .sup.7 Li, is vitally important for nuclear fusion furnaces and for nuclear fission furnaces.
To date, almost all attempt to enrich and separate .sup.6 Li have relied on by exchange reaction methods, followed by electrolysis methods, molecular distillation methods and ionic migration methods. All these methods make use of differences in reaction velocity or particle velocity due to the mass difference between the two isotopes.
The two-phase exchange reaction method which effects the enrichment by utilizing the phenomenon that the rates of penetration into two phases depend on the mass difference between the isotopes, necessitates use of a combination of two liquid phases which involve a large isotope separation factor. For example, in the Journal of Chemical Physics, Vol. 56, No. 5, pp 1855-1862 (1972), the combination of an organic solvent and a dielectric solvent is suggested. In the Journal of Chemical Physics, Vol. 57, No. 12, pp 5556-5561 (1972), the combination of an aqueous solution and an organic solvent is suggested. The combination of an aqueous solution and an amalgam is suggested in the Journal of Chemical Physics, Vol. 64, No. 4, pp 1828-1837. An ionic migration method which accomplishes the enrichment of lithium isotopes by utilizing the mobility of ions affected by the difference of masses of the lithium isotopes in a DC-applied solution passed therethrough has also been suggested [The Journal of Physical Chemistry, Vol. 62, 760 (1958)].
These methods invariably utilize the small mass difference only from the inertial point of view and, therefore, involve extremely small isotope separation factors. This means that acquisition of a desired isotope of lithium in a required quantity makes it necessary to perform the enrichment and separation process in a multiplicity of steps repeatedly for a long period of time. The ionic migration method consumes electric power in a quantity too large to make the operation thereof commercially feasible.
In the Japanese Patent Public Disclosure No. 13798/1968, there is disclosed a method whereby the isotopes of uranium are separated by irradiating uranium with a laser beam in two separate stages so as to excite and ionize the desired isotope. This method cannot be applied to the enrichment and separation of the isotopes of lithium, however, because uranium and lithium have entirely different spectrum structures and pressure characteristic and, accordingly, different shifts of absorption lines and different wavelengths, which are controlled by a entirely different method, of laser beams to be used.